Tom's Blog

Friday, December 4, 2015

Visualizing downed timber in an oak woods

Yesterday I had an unusual opportunity to see and photograph the downed timber present in our North Woods. There had been a light snow the day before, followed by a slight warming spell. This resulted in the melting of all the snow on the ground, but the snow on tops of downed logs did not melt. Thus, each log lying on the ground was clearly delineated, as the photo shows.

Photo taken from County F at approximately the middle of the North Woods.
Each log is clearly delineated because of its snow cover.

North Woods map (by GIS) showing distribution of tree species
The North Woods at Pleasant Valley Conservancy is a 25+ acre unit that has been generally undisturbed for many years. It consists primarily of red oaks (Quercus rubra), with substantial bur oak (especially at the top of the ridge and at the west end), plus Hill's and white oaks. The map above, based on our tree database. shows the distribution of all the larger trees.

Since management began at PVC about 20 years ago, this woods has been burned occasionally, recently on a 2-year burn interval. For the past three burns (six years) fire has carried very well, with coverage greater than 90%. Therefore, I was surprised to see so much downed timber yesterday, which should have blocked the movement of the fire.

It is interesting that almost every log is lying in parallel, horizontal with the slope. Since fire moves uphill, it would appear that its movement would be blocked. My conclusion is that with a substantial oak leaf litter, the flames are high enough so that they are able to skip over the downed logs and keep on moving. In addition, the fire crew was doing extensive internal lighting, thus insuring that the flame front would start anew whenever it gets blocked and dies. This is a major reason why "internal lighting" or use of the "strip headfire technique" is essential when conducting oak woods burns. As I keep reiterating, you can't use your prairie fire techniques when burning oak woods.

The question arises as to what is the source of all this downed timber? As the photo shows, most of the logs are of substantial size, although not as large as those standing. Because of the dense canopy (>90% cover), lower branches will be shaded and unable to continue growth. This is the standard "self-pruning" that occurs in a woods. There are probably other factors as well.

Oak wood does not decompose very quickly, so that lots of "carbon" ends up being stored on the forest floor. There is now quite a bit of research on carbon storage in woods, as part of climate change research.


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